import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class C1_11_10 {
    static int len;
    static int num;
    static int[] stones;
    static int output = 0;
    public static boolean ifOK(int mid) {
        int index = 0;
        int tot = 0;
        int now = 0;
        while (index < len + 1) {
            index++;

            if (stones[index] - stones[now] < mid) {
                tot++;
            } else {
                now = index;
            }
            if (tot > num) {
                return false;
            }
        }
        output = mid;
        return true;
    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String strTmp = br.readLine();
        int far = Integer.parseInt(strTmp.split(" ")[0]);
        len = Integer.parseInt(strTmp.split(" ")[1]);
        num = Integer.parseInt(strTmp.split(" ")[2]);
        stones = new int[len + 2];
        stones[len + 1] = far;
        stones[0] = 0;
        
        for (int i = 1; i <= len; i++) {
            String numStone = br.readLine();
            stones[i] = Integer.parseInt(numStone);
        }
        int left = 0;
        int right = far;
        while (left <= right) {
            int mid = (left + right) >>> 1;
            if (ifOK(mid)) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        System.out.println(output);
    }
}
